This invention relates to semiconductor wafers, and particularly to those having nitride semiconductors grown by epitaxy on a substrate of silicon, silicon compounds or the like. The invention also specifically pertains to semiconductor devices manufacturable from the wafers, such for example as high-electron-mobility transistors (HEMTs), metal semiconductor field-effect transistors (MESFETs), and light-emitting diodes (LEDs), and to a method of making such wafers.
The semiconductor wafer having layers of nitride semiconductors grown on a silicon substrate by epitaxy has been known, as disclosed for example in Japanese Unexamined Patent Publication No. 2003-59948. Silicon is preferred as a less expensive substitute for sapphire as a substrate material. However, a problem has been encountered in use of a silicon substrate in this type of wafer by reason of an inconveniently great difference in coefficient of linear thermal expansion between the silicon substrate and the nitride semiconductors grown thereon. The linear expansion coefficient of silicon is approximately 4.70×10−6/K whereas that of gallium nitride, for example, is 5.59×10−6/K. Other semiconducting nitrides are more or less equally higher in linear expansion coefficient than silicon. What is worse, silicon and semiconducting nitrides also differ in lattice constant. Therefore, grown directly on the silicon substrate, the nitride layers have been unavoidably stressed, with consequent development of cracks or dislocations therein.
A conventional remedy to this inconvenience, as taught by the Japanese patent application cited above, is an interposition of a multilayered buffer between the silicon substrate and the nitride semiconductor region. The multilayered buffer is designed to mitigate the stresses exerted on the nitride semiconductor region, protecting the same against cracking and dislocations.
This solution has proved unsatisfactory, however, particularly as semiconductor manufactures today are bent upon developing and using larger wafers for reduction of production costs. The wafers formed by growing the nitride semiconductor region on the silicon substrate via the buffer have proved to become increasingly more susceptible to warpage as the nitride semiconductor region grows thicker and, moreover, as the wafer increases in surface area or diameter. Thicker nitride semiconductor regions, however, have their own merit: They enable the resulting devices to withstand higher voltages in their thickness direction. Indeed, the thicker the nitride semiconductor region, the greater is the antivoltage strength in its thickness direction. Larger wafers are directly conducive to the curtailment of manufacturing costs, for a larger wafer yields a greater batch of devices than does a smaller one.
Another known method of growing a nitride semiconductor on a silicon substrate is found in “High Quality GaN Grown on Si(111) by Gas Source Molecular Beam Epitaxy with Ammonia” by Nikishin et al. in the volume 75, number 14 of Applied Physics Letters dated Oct. 4, 1999. Nikishin et al. teach a superlattice buffer between a silicon substrate and a main semiconductor region of GaN for providing the desired working parts of desired semiconductor devices. The superlattice buffer incorporates two superlattices each having alternating AlGaN and GaN layers, with an additional GaN layer interposed therebetween. An alternative method is reported in “Stress Control in GaN Grown on Silicon (111) by Metalorganic Vapor Phase Epitaxy” by Feltin et al. in the volume 79, number 20 of Applied Physics Letters dated Nov. 21, 2001. Feltin et al. employ AlN/GaN superlattices in lieu of the AlGaN/GaN superlattices of Nikishin et al.
The foregoing two prior art buffer configurations serve each in its own way to save the GaN layers from cracking and to improve their crystallinity. However, they are not explicitly designed for elimination of wafer warpage. It has indeed proved that they leave this problem unremedied, especially with wafers that must be made comparatively thick for higher antivoltage strength.